1. Field of the Invention
This invention is a fluid catalytic cracking process for converting an intermediate boiling point-range hydrocarbon-containing material to more valuable products and, in particular, is a method for producing an isolated catalytic cracking product having increased low molecular weight olefin and mono-aromatic contents and a reduced sulfur content and having a high octane rating, by catalytically cracking an isolated reactant feed, at least 80 volume percent of which is a substantially mono-aromatic hydrocarbon fraction boiling in the boiling range of heavy gasoline and the remainder of which is a vaporizable hydrocarbon fraction.
2. Description of the Prior Art
Since World War II, catalytic cracking has been the primary method for converting heavy components of crude oil into gasoline in the refinery. The product from catalytic cracking has been of suitable quality for direct economical blending to gasoline. However, the trend toward lower sulfur and lead levels in future gasolines has made some form of reprocessing of the product from catalytic cracking necessary. Requirements for additional sources of light olefins for either chemical feedstock or alkylate feedstock and of light aromatics for chemical feedstocks have also made reprocessing desirable.
There have been numerous attempts to reprocess heavy hydrocarbon fractions. For example, Hampton, U.S. Pat. No. 3,758,400 (1973) discloses a process for cracking in one catalytic cracking reactor a hydrocarbon feed oil, including a recycle oil, which is higher boiling than gasoline and which normally boils above about 600.degree. F., and is, in particular, recycled gas oil which was produced in a second catalytic cracking reactor.
Harper, U.S. Pat. No. 2,941,936 (1936) discloses a process which is adapted for the catalytic cracking of different types of hydrocarbons in separate catalytic cracking units, which involves cracking a substantially virgin gas oil in the presence of a catalyts, fractionating the product therefrom to obtain a fraction containing heavy cycle oils and decant oils containing some aromatics, cracking this fraction in the absence of virgin gas oil and in the presence of a catalyst, to thereby produce a product having substantially increased aromatics content, solvent extracting this product in order to obtan therefrom the aromatics therein and to return a raffinate thus obtained from such solvent extracting to the first zone.
Bunn et al., U.S. Pat. No. 3,448,037 (1969) discloses a method for a fluid catalytic cracking of hydrocarbon oils wherein a virgin gas oil and a cracked cycle oil which boils in the range of from about 430.degree. F. to 900.degree. F. and which comprises principally a stream boiling above gasoline, preferably a recycle stock, boiling between about 600.degree. and 800.degree. F. comprising intermediate cycle gas oils, are individually cracked through separate elongated reaction zones with a zeolite cracking catalyst under high conversion conditions, combining the zeolite catalyst from the individual elongated reaction zones into a dense phase fluidized bed and further converting the cracked cycle gas oil effluent in the bed.
Bunn et al., U.S. Pat. No. 3,433,733 (1969) discloses in the discussion of the prior art a process wherein two separate hydrocarbon streams in two separate risers are introduced into the reaction zone, one such hydrocarbon stream comprising a virgin gas oil cracking stock and the second such hydrocarbon stream comprising a stock having substantially different cracking characteristics, for example, a cycle gas oil -- such as that separated from the fluid catalytic cracking products -- or an extract from the solvent refining of a gas oil.
Rehbein, U.S. Pat. No. 2,827,422 (1958) discloses that it is usually profitable and desirable to recycle in the fluid catalytic cracking process at least a part of the gas oil separated from the product recovered from the fluid catalytic cracking operation and that the optimum recycle rate is about one-half that of the virgin feed oil.
Martin, U.S. Pat. No. 3,649,522 (1972) discloses a process for catalytically cracking a feed made up of fresh gas oil and the recycled product of a recycled fluid catalytic cracking product, with such product being about 30 volume percent of the fresh gas oil feed and having a boiling range, for example, of from about 480.degree. F. to about 750.degree. F.
Woertz, U.S. Pat. No. 2,890,164 (1959) and Hennig, U.S. Pat. No. 3,065,166 (1962) disclose in their discussions of the prior art that intermediate conversion cracking products are separated from the cracked products in a fractionation zone and are recycled back to the incoming charge-oil line where they are mixed directly with either partially reactivated catalyst or catalyst which has been subjected to complete regeneration and that, in still other methods, the intermediate boiling fractions from the cracked products are subjected to cracking in a separate cracking zone using a portion of regenerated catalyst and the products are either transmitted to a common fractionator or separately fractionated. In the method of Woertz, a heavy gasoline fraction boiling in the range of 350.degree.-425.degree. F. or light, recycled gas oil, which is separated from the fluid catalytic cracking products, is separately charged to a hot catalyst transfer line sufficiently far upstream from the point at which fresh feed is charged to the transfer line to accomplish substantial cracking of the heavy gasoline fraction or light, recycled gas oil between the points of feed entry, with about 180 moles of virgin gas oil being injected into the transfer line per hour and about 20 moles of recycle material per hour. The method of Hennig is similar to the method of Woertz but adds the step of separately hydrogenating the heavy gasoline fraction.
Haunschild, U.S. Pat. No. 3,803,024 (1974) discloses a process for catalytically cracking at least a portion of a fraction of fluid catalytic cracking products boiling above a given temperature with a second catalytic cracking catalyst in a second catalytic cracking zone.
Marshall, U.S. Pat. No. 2,921,014 (1960) discloses a method for the conversion of hydrocarbon oil boiling essentially above gasoline wherein the oil to be cracked is contacted in separate and distinct cracking stages under cracking conditions with a solid cracking catalyst and the product is removed by separate fractionation of the hydrocarbon reaction mixture from the separate cracking stages; the hydrocarbon mixture from the first stage of cracking is fractionated to remove as overhead product a hydrocarbon fraction having a final boiling point above about 266.degree. F. but below the final boiling point of the finished motor gasoline; the resulting bottoms product is subjected to further cracking in a subsequent catalytic cracking stage. The disclosed process permits recycle up to several, that is two or more, parts of recycle stock per part of fresh feed without loss in octane number.
McKinney et al., U.S. Pat. No. 3,692,667 (1972) discloses a process for cracking a primary fresh hydrocarbon charge capable of being cracked to lower boiling constituents including petrochemicals and the like in the presence of a stream of fluidized cracking catalyst involving recycling at least a portion of the normally liquid effluent product of the cracking process to the catalyst stream and adding the recycled portion to the catalyst stream at a point having a higher temperature than that at which the fresh charge is added so that a significant proportion of the recycle effluent is cracked by the catalyst. Typically, the recycle includes materials boiling between about 180.degree. F. and about 330.degree. or 375.degree. F. However, distillate fuel oils boiling from about 375.degree. F. to about 430.degree. F. can also be recycled. The quantity of recycle liquid can vary between about 5 and about 45 percent by volume based on the primary fresh hydrocarbon charge.
McDonald, U.S. Pat. No. 3,679,576 (1972) discloses a process for the catalytic cracking of hydrocarbons under fluid catalytic cracking conditions wherein the maximum efficiency is obtained from a catalyst by serially contacting various hydrocarbon feeds with the catalyst in an apparatus providing a plurality of riser reactors connected in series. One feed can be recycled material from other steps in the refining process and may have a boiling range varying over a wide range, for example, 450.degree. to 650.degree. F. for light oils and 650.degree. to 850.degree. F., or even higher, for heavy gas oils.
Reynolds et al., U.S. Pat. No. 3,761,394 (1973) discloses a process for catalytically cracking low octane naphthas with a zeolitic catalytic cracking catalyst wherein useful feedstocks include low octane naphthas from thermocracking or hydrocracking operations boiling in the range of 100.degree. to 450.degree. F.
Thus far, no one has disclosed the process of this invention.